1. Field of the Invention
The present invention relates to a mechanical filter, more particularly to a mechanical filter including at least one piezoelectric ceramic vibrator.
2. Description of the Prior Art
Mechanical filters are widely used as frequency selection devices which transform electric signals into ultrasonic waves as a transfer medium, and vice versa. Mechanical filters have numerous advantages, such as a high Q compared with coils and capacitors, maintained or increased stability, and compact construction. Because of these advantages, mechanical filters have been used as channel filters for channel translating equipment and as filters for a pulse code modulation (PCM) data transmission equipment used in, for example, the "information network service" (INS) of Japan and the "integrated service data network" (ISDN) now beginning to be constructed in numerous countries around the world.
A mechanical filter is usually fabricated with a vibrator, a coupler, and a support wire. The vibrator is most preferably made of a piezoelectric ceramic material. The prior art piezoelectric ceramic vibrator of the longitudinal vibration mode suffers from two basic problems. First, it exhibits no frequency response characteristics of the even number order modes, e.g., the second and fourth order mode. This means that a mechanical filter using a prior art piezoelectric ceramic vibrator has no filtering function under the second order mode, the fourth order mode, or higher even number order modes.
Second, while the prior art piezoelectric ceramic vibrator exhibits frequency response characteristics of the odd number order modes, e.g., the first, third, or fifth order mode, the frequency response is lower the higher the order. This means that a mechanical filter using a prior art piezoelectric ceramic vibrator has an insufficient filtering function under higher, odd number order modes.